1. Field of the Invention
The present invention relates generally to microscopic inspection methods and apparatus, and more particularly to an improved photomask and reticle inspection method and apparatus capable of automatically detecting and identifying extremely small defects in a photomask, or the like, by comparing portions of the inspected photomask to either other portions of the same mask or to data corresponding to similar portions stored in a pre-existing database.
2. Description of the Prior Art
It has long been known that defects in photomasks and other objects including arrays of repeated geometrical designs can be detected by comparing one portion of the array to a corresponding portion of another part of the array. For example, in the devices disclosed in the U.S. Pat. Nos. 4,247,203 and 4,347,001, to Keneth Levy et al, photomask defects are optically detected by simultaneously scanning two portions of a narrow strip of the same photomask which if without defect would be identical. Any differences between the two detections indicate that there is a possible defect in one of the mask portions, and such defect can be specifically located and identified. Although such devices have long provided reliable detection of mask defects, it has been found that the detection system itself may generate spurious signals which show up in the output as defects when no actual defect is present. These "false defects" may result from such things as slight differences in geometries of the inspected mask portion, differences in focusing of the images onto the detectors, differences in illumination, vibrations of the system, misalignment of the inspected image and the reference image, intermittent memory bits and other similar problems.
An effort to provide a solution to the false defect problem is disclosed in U.S. Pat. No. 4,448,532 issued to David A. Joseph et al. In the disclosed apparatus, photomask defects are likewise optically detected by simultaneously scanning two portions of a narrow strip of the same photomask. However, "false defects" are avoided by rescanning each scan line if a defect is recorded, comparing the outputs thereof and discarding those detected defects which are not common to the two scans. More specifically, if no defect is found on the first scan then there is no reason to re-scan the line. But if defects are detected, the same line is re-scanned in the reverse and comparing the results of the two scans are compared and non-matching defect indications are discarded. Only the common defects are identified as real defects.
As the technology improved, it was found that prior inspection systems had relatively poor detection efficiency for defects located near corners. One attempt to overcome such in-efficiency was to use some kind template matching technique. However, there almost always existed cases involving unusual quantization in which errors occurring at or near image corners were missed.
Still another attempt to improve the detection efficiency and accuracy of photomask inspection systems is disclosed in the copending U.S. patent application of Kenneth Levy et al, Ser. No. 492,658, filed May 9, 1983 now U.S. Pat. No. 4,579,455. In accordance with this approach an area subtraction technique is used identify defects as differences between otherwise duplicate die patterns in a photomask. Two square window matrices of seven rows and seven columns of adjacent pixels are defined for corresponding areas of two die patterns of a single photomask (or a die pattern of a photomask and corresponding data taken from a pre-recorded database). The center 3.times.3 matrix of each window matrix is defined as a comparison matrix with each matrix having twenty-five unique subsets of 3.times.3 adjacent pixels within its boundaries; one in the center plus twenty four others that are offset by one or two pixels from the center in one or both directions. An error value is then calculated for each subset of each window matrix by summing the squares of the differences between each of the nine pixels values of each subset and the corresponding pixel values of the opposite comparison matrix. If there is no defect, and misalignment between the two representations is less than approximately two pixels in magnitude, at least one error value will be less than a threshold error value. If none of the twenty-five error values relating to one comparison matrix are less than the threshold value, a defect is assumed to be located within the comparison matrix or within the opposite window matrix. The magnitude of the threshold error is then automatically varied according to the number of edges within the window matrices to compensate for errors caused by different quantization of edges.
Among the advantages of this approach is that it dynamically and accurately inspects the photomask by identifying defects without requiring two perfectly aligned pixel representations. Furthermore, the sensitivity level of the defect detection circuit is made adaptive so as to improve actual defect detection while reducing false defect detection.
Although the foregoing devices have provided reliable detection of mask defects and have substantially reduced the false defect problem, it has remained desirable to improve the detection device even further to provide electronic correction for critical dimension variation, data collection variation, skew variation, magnification variation, and alignment variation. It has also remained desirable to provide improved sensitivity to defects regardless of the surrounding geometry, and in particular, to defects occurring near corners.
Each of the above mentioned patents and the patent application are assigned to the assignee of the present invention and the entire disclosure of each is expressly incorporated by reference into this application for teaching purposes.